The present invention is related to a durable magnetorheological (MR) fluid having improved performance upon exposure to a magnetic field.
Magnetorheological (MR) fluids are substances that exhibit the rather unique property of being able to reversibly change their apparent viscosity through the application of a magnetic field. For a MR fluid, the apparent viscosity, and related flow characteristics of the fluid, can be varied by controlling the applied magnetic field. These fluids have wide application in vibration dampening devices such as, for example, shock absorbers, vibration dampers, force/torque transfer (clutch) devices, and the like, and especially in systems in which variable control of the applied dampening/force is desirable.
Generally, MR fluids are suspensions of magnetizable particles in a carrier fluid. The particles are typically selected from iron, nickel, cobalt, and their magnetizable alloys. The carrier fluid is typically selected from mineral oil, synthetic hydrocarbon, water, silicone oil, esterified fatty acid or other suitable organic liquids. It is known that, over time, the carrier fluid may cause any seals associated with the vibration dampening device to shrink. Therefore, the carrier fluid is somewhat detrimental to the vibration dampening device in that it causes the seals to degrade over time and seal material is lost. The MR fluids of the prior art have not been able to combat this seal degradation.
MR fluids generally also further include a thickener, i.e., a thixotropic agent, to control settling, a phosphorus- or sulfur- containing additive to inhibit wear, and an organomolybdenum additive to inhibit friction. A surfactant may also be added to promote dispersability of the particles in the suspension.
The anti-wear additives used in prior art MR fluids have generally been selected from well-known anti-wear additives used in, for example, engine lubricants. These include thiophosphorus additives such as zinc dialkyl dithiophosphate (ZDDP). U.S. Pat. No. 5,683,615, for instance, describes a MR fluid comprising magnetic-responsive particles, a carrier fluid and at least one thiophosphorus or thiocarbamate; and U.S. Pat. No. 5,906,767 describes a MR fluid comprising magnetic-responsive particles, a carrier fluid and at least one phosphorus additive. Neither patent, however, discloses or suggests the use of a lubricative additive, such as polytetrafluoroethylene, graphite, and/or molybdenum disulfide, having a particle size equal to or less than 10 microns such that the additive can optimally lubricate the magnetizable particles and inhibit wear.
The anti-friction additives used in prior art MR fluids have also been generally selected from well-known organomolybdenum compounds used as anti-friction additives in engine lubricants. For example, U.S. Pat. No. 5,705,085 describes a MR fluid that includes magnetic-responsive particles, a carrier fluid and an organomolybdenum; and U.S. Pat. No. 5,683,615 also describes the use of the same organomolybdenum compounds in the MR fluids disclosed. Neither patent, however, discloses or suggests the use of a lubricative additive, such as polytetrafluoroethylene, graphite, and/or molybdenum disulfide, having a particle size equal to or less than 10 microns such that the additive can optimally lubricate the magnetizable particles and inhibit friction.
Furthermore, the anti-wear and anti-friction additives commonly used in prior art MR fluids such as ZDDP and organomolybdenum compounds pose environmental hazards due to the presence of heavy metals. Therefore, more environmentally friendly anti-wear and anti-friction additives are preferred, but as yet, have not been found.
Due to the deficiencies in the MR fluids of the prior art, including those described above, it is desirable to provide a MR fluid that is durable, that combats shrinkage and degradation of seals, and that substantially replaces the phosphorus- and sulfur-containing anti-wear additives and the organomolybdenum-based anti-friction additives found in prior art MR fluids.
The subject invention provides a durable MR fluid. The MR fluid includes magnetizable particles, a carrier fluid, a thixotropic agent, and a lubricative additive. More specifically, the magnetizable particles have a particle size less than about 25 microns, and the carrier fluid includes a polyalphaolefin and a plasticizer. The lubricative additive includes at least one of polytetrafluoroethylene, graphite, and molybdenum disulfide. Furthermore, the additive has a particle size equal to or less than 10 microns such that the ability of the additive to lubricate the magnetizable particles is optimized.
The MR fluid of the subject invention has anti-wear and anti-friction properties, but does not contain the heavy metals known in the prior art to pose environmental hazards. The lubricative additive gives the MR fluid of the subject invention its anti-wear and anti-friction properties. Therefore, the lubricative additive substantially replaces the phosphorus-based anti-wear additives and the organomolybdenum-based anti-friction additives found in prior art MR fluids. The lubricative additive inhibits surface-to-surface contact and scuffing within a vibration dampening device that utilizes the MR fluid while providing reliable lubrication under boundary lubrication conditions. Because the lubricative additive has a particle size equal to or less than 10 microns, the additive is more easily incorporated into the carrier fluid, the additive can penetrate into the microscopic crevices of the vibration dampening device, and lubrication of the magnetizable particles is optimized.
The plasticizer utilized in the carrier fluid combats shrinkage and degradation of any seals in the vibration dampening devices that utilize the MR fluid of the subject invention. More specifically, it has been found that the plasticizer provides seal swell. That is, the incorporation of the plasticizer in the carrier fluid causes any seals in the vibration dampening device to swell thereby regulating the integrity of the seal, or seals, over time and insuring against the loss of any seal material that would normally result from wear.